Mechanisms of Sodium Transport in Plants—Progresses and Challenges

Understanding the mechanisms of sodium (Na⁺) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na⁺ accumulation and assure the maintenance of low Na⁺ concentration in the cytosol and, hence, plant tolerance to salt stress. Na⁺ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na⁺ is compartmentalized into vacuoles by Na⁺/H⁺ exchangers (NHXs). Na⁺ efflux from the plant roots is mediated by the activity of Na⁺/H⁺ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na⁺, K⁺-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na⁺, K⁺-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na⁺ influx, compartmentalization, and efflux in higher plants in response to salt stress.